The study of skill learning has, like many other areas of memory research, been influenced by the implicit/explicit distinction. Current thinking is that implicit and explicit processes are separate, but both can contribute to skilled behavior. The implicit/explicit distinction has been useful, but it has perhaps led to an overemphasis on the separability of implicit and explicit processes. This proposal addresses the question: Do implicit and explicit processes interact in skill learning? The interaction has been hard to address because implicit learning must be assessed through performance, but explicit learning can affect performance too. In the past researchers tried to ensure that no explicit learning occurred, so that implicit learning would not be "corrupted" by the contribution of explicit processes. We have developed techniques that allow the assessment of implicit learning in isolation, even if explicit learning has taken place. We can assess implicit learning in isolation for a motor skill (sequence learning) and a cognitive skill (probabilistic categorization). Thus, we can train subjects explicitly and later examine how this training affected implicit learning. The specific questions about system interaction are inspired by four principles of system interaction in the rat. (1) There are anatomically separate systems in amygdala, hippocampus, and striatum supporting different types of learning; (2) These different types of learning occur in parallel; (3) These different systems learn different types of information; (4) These different systems can either cooperate or compete, perhaps because each system learns information that potentiates complimentary or conflicting behaviors OR because of direct anatomic connections. There is excellent evidence for separate systems in humans, but limited evidence on the other three points. The motivation of this proposal is to test system interactions using the findings from rats as a starting point. Our specific aims are to test (1) whether implicit and explicit learning occur in parallel; (2) whether implicit and explicit learning use different representations; (3) whether cooperation/competition among memory systems depends on the compatibility of what each system learns. This work has potentially important implications for rehabilitation (e.g., from stroke). Both motor loss and cognitive loss is often treated via explicit training in the hopes that the explicit training will influence implicit processes.